In terms of chords and melody, Hudson's Adventure Island IV NES - Unknown Theme A is more complex than the typical song, having above average scores in Chord Complexity, Melodic Complexity, Chord-Melody Tension and Chord Progression Novelty.
Hudson's Adventure Island IV NES - Unknown Theme A is written in the key of A♭ Mixolydian. According to the Theorytab database, it is the 9th most popular key among Mixolydian keys and the 41st most popular among all keys. The A♭ Mixolydian scale is similar to the A♭ Major scale except that its 7th note (G♭) is a half step lower. Mixolydian chord progressions are heavily featured in many genres of music like classic rock, which relies on the major chord built on the 7th scale degree. See the A♭ Mixolydian Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!